Numerous metals or metallic compounds catalyze the hydrogenation of coal to liquids and gases. These materials are especially effective in promoting the conversion of coal to liquids during hydrogenation at temperatures in the range from 850.degree. to 1050.degree. F. in the absence of a pasting oil. At higher temperatures hydrocarbon liquids which are formed tend to be rapidly cracked to gases.
It should be noted, however, that catalysts may be used effectively in processes in which the major product is a high-Btu gas, such as intended for pipeline transmission. The presence of the catalyst is effective in converting the coal in the hydrogenation reactor primarily to methane and ethane. Operating temperatures in the reactor to produce methane and ethane will generally be in the range from 1050.degree. to 1350.degree. F.
The catalysts for oil or gas production are most effective when they are well dispersed so that the catalyst, coal particle and hydrogen are in simultaneous juxtaposition. This promotes the formation of dissociated hydrogen at the location where it is most effective in hydrogenating the coal to liquid or gas.
Various methods are used to secure uniform dispersion of the catalyst on the coal particles and in fact to have the catalyst penetrate the interstices of the coal particle insofar as possible. These include dissolving or dispersing the catalyst in a liquid in which the crushed or pulverized coal is then immersed. Dissolved catalyst may also be sprayed on crushed coal, the coal then being pulverized to obtain good distribution
Metals which are known to form effective catalytic compounds include cobalt, iron, manganese, molybdenum and nickle. Some may be used in the metal form but are more frequently used as oxides, sulfides, or chlorides which may be soluble in water or chemical solutions.
Efficient catalyst recovery is particularly essential for hydrogenation processes using catalytic compounds of such relatively expensive metals as cobalt, molybdenum or nickel. For example, it is desirable to have approximately 1 weight percent molybdenum present during the hydrogenation of pulverized coal. At present prices for molybdenum compounds, 96 percent of this metal must be recovered and recycled to hold catalyst make-up costs to approximately $1.00 per barrel of oil equivalent produced.
In coal hydrogenation processes, gases and light oil vapors may easily be separated from a carbonaceous residue containing heavy oils-tars, unreacted coal and carbon, as well as ash. The majority of the metallic compounds used as catalysts revert to solids under hydrogenation conditions and remain with the carbonaceous residue. Accordingly, if the carbonaceous residue containing ash is sent to a synthesis gas generator to produce H.sub.2 and CO for the process, the catalyst also goes to the synthesis gas generator.
The direct recovery of molybdenum catalyst by vaporization of molybdenum oxide in a synthesis gas generator has been suggested (U.S. Pat. No. 3,729,407, Apr. 24, 1973, Frederick W. Camp et al). However, under conditions existing in a synthesis gas generator, i.e. high pressures of hydrogen and CO at a temperature of 1500.degree. F. to 3000.degree. F., essentially all of the molybdenum oxide would be reduced to the metal, which has a melting point of 4740.degree. F., and the metal would not be present as a vapor. It is evident that most of the metal would be entrapped in the ash and would leave the synthesis gas generator with the ash, and therefore the method of catalyst recovery suggested in this patent is not very effective for molybdenum compound recovery.